Is there a way to make the active object semi-transparent in sculpt mode?
It looks like "a solution" is to


Assign a new material to the active object.
Add a Transparent BSDF to the Diffuse BSDF via a Mix Shader and tweak
the Mix Shader's Fac value.
Go to the Material tab>Settings>Viewport Alpha and set it to Alpha
Blend.
Go to the Object tab>Display and enable X-Ray.
Go to Sculpt Mode and set the Viewport Shading to Material.




The white object is the active object that is being sculpted.

Note: You will have to manually assign this material to the active object and unassign it from the inactive objects as part of your workflow, in addition to enabling X-ray for the active object and disabling X-ray from the inactive object. An add-on that could do this would be very nice.